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US4539674A - Method of providing adaptive echo cancellation in transmission of digital information in duplex, and apparatus for performing the method - Google Patents

Method of providing adaptive echo cancellation in transmission of digital information in duplex, and apparatus for performing the method Download PDF

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Publication number
US4539674A
US4539674A US06/514,825 US51482583A US4539674A US 4539674 A US4539674 A US 4539674A US 51482583 A US51482583 A US 51482583A US 4539674 A US4539674 A US 4539674A
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Prior art keywords
signal
transmitter
sampled
filter
receiver means
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US06/514,825
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English (en)
Inventor
Bengt R. Carlqvist
Lars T. E. Svensson
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Telefonaktiebolaget LM Ericsson AB
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Telefonaktiebolaget LM Ericsson AB
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Assigned to TELEFONAKTIEBOLAGET L M ERICSSON S-126 25 STOCKHOLM, SWEDEN A CORP. OF SWEDEN reassignment TELEFONAKTIEBOLAGET L M ERICSSON S-126 25 STOCKHOLM, SWEDEN A CORP. OF SWEDEN ASSIGNMENT OF ASSIGNORS INTEREST. Assignors: CARLQVIST, BENGT R., SVENSSON, LARS T. E.
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    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04B—TRANSMISSION
    • H04B3/00—Line transmission systems
    • H04B3/02—Details
    • H04B3/20—Reducing echo effects or singing; Opening or closing transmitting path; Conditioning for transmission in one direction or the other
    • H04B3/23—Reducing echo effects or singing; Opening or closing transmitting path; Conditioning for transmission in one direction or the other using a replica of transmitted signal in the time domain, e.g. echo cancellers
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04B—TRANSMISSION
    • H04B3/00—Line transmission systems
    • H04B3/02—Details
    • H04B3/20—Reducing echo effects or singing; Opening or closing transmitting path; Conditioning for transmission in one direction or the other
    • H04B3/23—Reducing echo effects or singing; Opening or closing transmitting path; Conditioning for transmission in one direction or the other using a replica of transmitted signal in the time domain, e.g. echo cancellers
    • H04B3/231—Echo cancellers using readout of a memory to provide the echo replica

Definitions

  • the present invention relates to a method of providing adaptive echo cancellation in transmission of digital information in duplex over a single pair of conductors.
  • the invention also relates to apparatus for performing the method.
  • Adaptive echo cancellation is provided in telephony and data communication engineering to prevent echo signals from affecting the reception.
  • the transmitter in one of the two-wire directions and the receiver in the other are connected to the two-wire line via a hybrid coupler, data (b k ) being sent from the transmitter across the line to a modem at the remote end, while conversely, data (a k ) is transmitted from the remote end across the line to the local modem receiver.
  • Due to deficiencies in the hybrid coupler it is unavoidable that a certain proportion of data flow (b k ) from the transmitter passes through the hybrid coupler and into the receive path, reception of the data flow (a k ) thus being disturbed.
  • a digital-type balance filter usually a finite impulse response (FIR) filter connected to the transmit and receive channels.
  • the task of the balancing filter is accordingly to form a signal from the transmitting data flow, this signal being subtracted from the one which occurs at the detector input, after having passed through the hybrid coupler, and which contains leak signals from the transmit channel.
  • FIR finite impulse response
  • the balance filter parameters i.e. provide rapid convergence in the balance filter, it is however required that the correlation between the incoming analogue signal in the receive path (denoted below w(t)+h(t)) and the transmitted data flow b k be great.
  • the presence of the remote signal w(t) coming from the data flow (a k ) decreases this correlation however, and convergence is slow.
  • Rapid convergence of the balance filter thus requires that the analogue remote signal w(t) be eliminated in some way when calculating the parameter adjustment.
  • a known method of eliminating or cancelling signal w(t) is to ensure that the remote end transmitter is disconnected during a test period when the adjustment of the balance filter can be carried out. Transmission quality will then be entirely dependent on how successful the adaption has been during the test period. The test period must be made relatively long. It would be more effective if signal w(t) could be subtracted from incoming signal w(t)+h(t). In digital transmission signal w(t) only attains a limited number of amplitude values at the sampling instants (if intersymbol interference is neglected). An estimation w(k) of signal w(t) can therefore be made in the sampling instants k with the aid of a quantizer.
  • An object of the present invention is to provide a method for adjusting a digital-type balance filter, e.g. one incorporated in a data modem with rapid convergence, without special test periods and without needing to resort to adaptive level adjustments.
  • the inventive method contemplates forming, from the data flow including both remote signal w(t) and echo signal h(t) coming into the modem, correction signals to the balance filter only when the level of the incoming flow exceeds a given reference value, so that the filter is only corrected when the incoming signal level exceeds the reference value. This contemplates disconnecting the remote signal, and rapid convergence (apart from an acceptable residue error) of the balance filter is enabled.
  • FIG. 1 is a block diagram of a data modem utilizing the method in accordance with the invention
  • FIG. 2 is a time diagram of a remote signal
  • FIG. 3 depicts in a time diagram examples of a binary data flow transmitted from the remote end, corresponding to a biphase coded signal and an echo signal to the modem receive side,
  • FIG. 4 depicts in a diagram sampled values from the echo signal mentioned
  • FIG. 5 is a diagram of sampled values ⁇ 1 from the remote signal on reception
  • FIGS. 6-7 are diagrams of sampled values from the echo signal and remote signal in comparison with a variable reference level
  • FIG. 8 is a block diagram over a preferred embodiment of an apparatus in accordance with the invention.
  • a data sequence b k comprising ones alternating with zeroes is sent from an unillustrated data source, e.g. a telephone exchange, to a transmitter unit S including a code converter for conversion of the binary flow b k , e.g. to biphase coded (analogue) signals s(t), see FIG. 3.
  • the transmitter unit S is connected to a hybrid coupler G of known kind, which feeds the biphase coded signal s(t) out onto a two-wire line L.
  • the apparatus illustrated in FIG. 1 may be a part of a modem for data transmission, the outgoing data being the binary flow b k .
  • the data sequence coming into the modem from the remote end of the line L is denoted a k , and is transmitted from the remote end across the line L simultaneously as the data sequence b k is transmitted from the transmitter unit S across the line L to the remote end.
  • the flow a k from the remote end occurs in this assumed case as an analogue signal w(t) (biphase coded) across the input terminal of the hybrid coupled G from the line L.
  • w(t) biphase coded
  • a balance filter B suitably a FIR filter, is connected across the input to the transmitter unit S and to a minus input of the summing circuit A1.
  • the data flow b k together with the corresponding biphase coded signal s(t) after the transmitter S and the leak signal h(t) are shown in FIG. 3.
  • the signal h(t) derived from the signal s(t) from the transmitters is heavily correlated to this signal and, at least at the beginning of transmission, is very much stronger at the input to the lowpass filter LP than the remote signal w(t).
  • the balance filter B should thus send a signal y(t) which is also heavily correlated to the signal s(t).
  • the input signals r(k) to the detector or quantizer Q will be independent of the transmitted data sequence b k at the sampling instants t k , i.e. y(t k ).
  • the detector Q is a quantizing circuit which, for the sake of simplicity, should be considered here as comprising a comparator deciding whether the sampling values r(k) are greater or less than zero.
  • the quantity a k across the output of the quantizer Q is +1 if r(k)>0 or -1 if r(k)>0, and gives an estimation of the data a k sent from the remote end.
  • the block V is a reference unit in which a reference value is stored in digital form and may include a digital-to-analogue converter.
  • the output of the unit is connected to one input of a multiplier M, the other input of which is connected to the output of the quantizer Q.
  • One input of a second summing circuit A2 is connected to the output of the multiplier, and the other input of the second summing circuit is connected to the input of the quantizer Q.
  • the output of the summing circuit A2 is connected to a sign-forming circuit SG.
  • the output of this is connected to a third summing circuit A3 and to one input of a correction unit KV for correcting the reference value stored in the reference unit V.
  • the correction unit KV is connected by its second input to the output of the quantizer Q and may be a multiplier, for example.
  • a third input to the correction unit KV connected to the reference unit V is supplied with the reference value from this unit and the new, possibly corrected value is supplied to the reference unit V via the output of the correction unit KV whose output is connected to the reference unit V.
  • the reference levels in the reference unit V may possibly control the levels in the quantizer Q as indicated by the dashed connection.
  • the sign of e k is obtained across the output of the circuit SG, and it may thus be ⁇ 1.
  • the sign of the error signal e k is correlated with the value a k in the correction unit KV, which computes a correction for adding to v(k).
  • the sign of e k obtained across the output of the unit SG denotes whether the value a k ⁇ v(k) was greater or less than the sampled value r(k).
  • the value r(k) in turn denotes how near the received signal r(t) the received signal w(t) comes, after having passed the hybrid coupler (addition of h(t)) and after the subtraction of the signal y(t) from the balance filter.
  • the sign of e k is summed in the summing circuit A3 to a k , and is supplied via a first input to a second correction unit KB for the balance filter B. There is furthermore supplied to this unit, via a second and a third input, the reference level v(k) and the balance filter parameters cj(k).
  • the correction unit KB then calculates the new parameters cj(k+1 ) of the filter B, which occur on the output connected to a control input of the filter B.
  • Correlation of the quantity ⁇ k sign e k +a k with the data flow b k is performed in the correction unit KB (a correlator), the relationship between the estimated value of the remote signal w(t) represented by sign e k +a k and the transmitted data flow b k is established. If the correction unit KB finds that the quantities ⁇ k and b k are correlated, i.e. that there is an undesirable relationship between received data a k and data flow b k , the parameters c j of the balance filter will be corrected so that this correlation decreases. On the other hand, if the magnitudes ⁇ k and b k are uncorrelated there is no relationship between a k and b k . The balance filter may then be considered converged and the corrections will not change the adjustment of the parameters c j .
  • FIG. 2 illustrates the relationship between the sampled received signal r(k), the remote signal w(t), the reference level v(k) and the error signal e k .
  • the balance filter B is of the FIR type, i.e. the output signal in every instant is solely dependent on a limited number N samples of the input signal, i.e. the data sequence b k .
  • the filter function is determined by its parameters c 1 . . . c M and N consecutive input signal values b k , b k-1 . . . b k-N+1 the (digital) output signal ##EQU1## and y(t) is the corresponding analogue signal.
  • the digital output signal y k will then be the coefficient c j (k).
  • FIGS. 4-7 illustrating the case of a transmitted data flow b k from the transmitter S which consists of alternating +1's and where solely one coefficient c 1 in the filter B is updated.
  • the situation with the proposed method is that if the absolute value of the signal values r(k) is greater than a given reference level v(k), both reference level shall be increased and the coefficient c in the balance filter changed, whereas if
  • the balance filter then converges, according as the coefficient c increases and v(k) increases (the error signal e.sub. k becomes less and less).
  • the effect on the balance filter convergence being dependent on the remote signal has thereby been cancelled, and thereby also the above-mentioned problem with convergence of the balance filter.
  • the transmitter unit S, hybrid coupler G and lowpass filter LP have been excluded.
  • the signal w(t)+h(t) is sent from the lowpass filter to the summing circuit A1, the other input of which receives a signal y(t) from the output of a digital-to-analogue converter DA1.
  • this comprises an operational amplifier Q working as a comparator, its plus input being connected to the sampling circuit SH and its minus input being connected to the reference potential 0 (ground).
  • the comparator Q thus decides whether the sampled signal's levels are greater or less than zero.
  • Such an implementation of a quantizer can be used if only two levels of the incoming signal w(t)+h(t) are to be detected.
  • the output magnitude a k from the comparator Q is thus still 0 or 1.
  • the apparatus in accordance with the invention further includes: the reference unit V, having a memory unit MV, a digital-to-analogue converter DA2 connected to the output of the memory unit MV and a correlator KV including logic circuit E3 and a summing circuit A4 and two multipliers M3 and M4.
  • a reference value v(k) is stored in the memory unit MV and is utilized in both comparators J1 and J2, after conversion to its analogue value in the converter DA2.
  • the value v(k) can be updated in the memory unit MV via a write input connected to the output of a summing circuit A4.
  • One input of the summing circuit A4 is connected to the-output of the memory unit MV and the other to the output of the multiplier M4.
  • the multiplier M3 connected to one input of the multiplier M4, forms a value ⁇ vk which, in certain logical conditions, occurs on the output of the multiplier M4 and thereby gives a new value ( ⁇ +1)v(k) for writing into the memory unit MV.
  • the comparator units J1 and J2 are each connected with their minus and plus inputs to the output of the digital-to-analogue converter DA2, and with their plus and minus inputs connected to the output of the sampling circuit SH, across which the quantity r(k) occurs.
  • An inverter circuit I1 is connected between the plus input of the comparator J2 and the converter DA2.
  • the outputs of the comparators Q and J2 are connected to an AND circuit O1 with inverting inputs, the output of which is connected to one input of an OR circuit E1, the second input of which is connected to the comparator J1.
  • the comparators J1 and J2 thus compare the sample values r(k) with the analogue value +v(k) and -v(k), respectively, i.e.
  • a sign-forming circuit which comprises two AND gates O2 and O3, an exclusive-OR circuit E2 and three controllable switches K1-K3 for sending a +1, 0 or -1 state in response to the binary states at the respective gate outputs.
  • the outputs of the switches are mutually connected to form the output of the sign-forming circuit.
  • This output is connected to one input of a first multiplier M1, the second input of which is connected to a second multiplier M2.
  • the value v(k) from the memory unit MV in the reference unit occurs at one input of the multiplier M2, and at the other input a constant value ⁇ (in digital form).
  • the outputs of the units K1-K3 are connected to one input of a multiplier M5, its other input receiving the data flow b k (cf FIG. 2).
  • a summing circuit A5 is connected by one input to the output of the multiplier M1 and by its other input it is connected to the output of a coefficient memory CM in the balance filter B.
  • This may be an FIR filter, which is a known kind of memory filter.
  • the filter B contains a shift register SB with N positions, an address register AE to form an address from the N values in the positions, and a coefficient memory CM.
  • the shift register SB stores the values b k , b k-1 , . . . b k-N+1 . These values give an integer presentation a(b k ).
  • An address j a(b k ) is formed in the address register AE, to point out a given coefficient c j (k) in the coefficient memory CM.
  • An array of digital values c j is obtained across the output of the balance filter, these values being the signal y(t) via the converter DA1.
  • the output of the summing circuit A5 is connected to an input of the memory CM for updating the coefficients c j (k) to c j (k+1).
  • the signal y(t) is added to the incoming analogue signal w(t) from the lowpass filter to form the signal r(t).
  • the operation of the apparatus will now be described in detail with reference to the time diagram according to FIG. 3.
  • the binary data flow (b n ) transmitted from a data source (telephone exchange) to the transmitter unit S is illustrated in FIG. 3.
  • the transmitter may include a conventional coding unit, which may convert the data flow b n to biphase code and a corresponding analogue biphase coded signal s(t) is shown in FIG. 3.
  • a part of the signal s(t) will pass through the coupler G and be added as the signal h(t) to the analogue biphase coded signal w(t) across the line L.
  • each of the comparators Q and J1, J2 give 1 if the level at the associated plus input is greater than the level over the minus input, and 0 if the level is less the following truth can be given:
  • the signal value r(k) is greater than the reference level v(k), resulting in that it will be increased by a factor (1 + ⁇ )v(k). Furthermore, the filter parameters c j will be corrected by a factor ⁇ v(k).
  • a residue error remains after convergence of the balance filter, the size of this error being in proportion to the size of the step lengths used in updating the parameters c j (k). For higher received r(k) signal levels, a larger residue error than for lower ones may be tolerated. Since the reference value v(k) is a measure of the signal strength of r(k), the convergence may be further accelerated if the step length is controlled by the reference value.
  • a dashed connection has been denoted between the units V and Q, i.e. quantizing of the incoming sample values r(k) is dependent on the reference level v(k). At the start of the adjustment, r(k) is typically dominated by the contribution from the local transmittter S, i.e.
  • the reference value v(k) then adapts itself to a level determined by the sample h(k) and the adaption is carried out with great step length. By degrees, as the convergence continues, r(k) diminishes and thereby the reference value v(k) until the balance filter B is completely converged, with the exception of an acceptable residue error.

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  • Engineering & Computer Science (AREA)
  • Computer Networks & Wireless Communication (AREA)
  • Signal Processing (AREA)
  • Cable Transmission Systems, Equalization Of Radio And Reduction Of Echo (AREA)
  • Filters That Use Time-Delay Elements (AREA)
  • Bidirectional Digital Transmission (AREA)
  • Analogue/Digital Conversion (AREA)
US06/514,825 1981-11-02 1982-11-02 Method of providing adaptive echo cancellation in transmission of digital information in duplex, and apparatus for performing the method Expired - Lifetime US4539674A (en)

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SE8106444 1981-11-02
SE8106444A SE426764B (sv) 1981-11-02 1981-11-02 Forfarande att astadkomma adaptiv ekoeliminering vid overforing av digital information i duplex jemte anordning for utforande av forfarandet

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EP (1) EP0092570B1 (es)
JP (1) JPS58501850A (es)
KR (1) KR860002211B1 (es)
AU (1) AU553372B2 (es)
BR (1) BR8207953A (es)
CA (2) CA1205876A (es)
DE (1) DE3265650D1 (es)
DK (1) DK306483A (es)
ES (1) ES8402483A1 (es)
FI (1) FI74561C (es)
IE (1) IE53759B1 (es)
IT (1) IT1153338B (es)
MX (1) MX152441A (es)
MY (1) MY8600355A (es)
NO (1) NO157238C (es)
SE (1) SE426764B (es)
WO (1) WO1983001716A1 (es)
YU (1) YU244082A (es)

Cited By (8)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US4631475A (en) * 1982-12-21 1986-12-23 Telefonaktiebolaget Lm Ericsson Method of detecting hazardous line voltages over a two-wire line
US4686703A (en) * 1984-03-26 1987-08-11 Cselt-Centro Stude E Laboratori Telecomunicazioni S.P.A. System for bidirectional digital transmission with echo cancellation
US4695998A (en) * 1983-07-29 1987-09-22 Siemens Aktiengesellschaft Method and circuit arrangement for the compensation of echo signals
US4707824A (en) * 1983-12-15 1987-11-17 Nec Corporation Method and apparatus for cancelling echo
US4989221A (en) * 1987-03-30 1991-01-29 Codex Corporation Sample rate converter
US5062102A (en) * 1988-12-01 1991-10-29 Nec Corporation Echo canceller with means for determining filter coefficients from autocorrelation and cross-correlation coefficients
US5852661A (en) * 1995-02-17 1998-12-22 Advanced Micro Devices, Inc. Adaptive echo cancellation used with echo suppression to reduce short and long duration echoes
US6965578B1 (en) * 1998-09-30 2005-11-15 Conexant Systems, Inc. Echo canceling method and apparatus for digital data communication system

Families Citing this family (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
FR2540314A1 (fr) * 1983-01-31 1984-08-03 Trt Telecom Radio Electr Procede d'initialisation des coefficients de filtres dans un dispositif d'annulation d'echos proche et lointain et dispositif de mise en oeuvre de ce procede
JPS59151546A (ja) * 1983-02-18 1984-08-30 Nec Corp 適応形反響消去装置
SE441052B (sv) * 1984-02-01 1985-09-02 Ellemtel Utvecklings Ab Forfarande for att testa funktionen hos en anordning for adaptiv ekoeliminering
US4672665A (en) * 1984-07-27 1987-06-09 Matsushita Electric Industrial Co. Ltd. Echo canceller

Citations (2)

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Publication number Priority date Publication date Assignee Title
US4074086A (en) * 1976-09-07 1978-02-14 Bell Telephone Laboratories, Incorporated Joint adaptive echo canceller and equalizer for two-wire full-duplex data transmission
US4425483A (en) * 1981-10-13 1984-01-10 Northern Telecom Limited Echo cancellation using transversal filters

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FR2277470A1 (fr) * 1974-07-03 1976-01-30 Cit Alcatel Dispositif de centrage d'un filtre transversal d'un annuleur d'echo
NL170688C (nl) * 1976-06-28 1982-12-01 Philips Nv Inrichting voor simultane tweerichtingsdatatransmissie over tweedraadsverbindingen.
NL7800408A (nl) * 1977-01-17 1978-07-19 Trt Telecom Radio Electr Digitale echocompensator voor een modem voor datatransmissie met behulp van modulatie van een draaggolf.
DE2729109A1 (de) * 1977-06-28 1979-01-11 Siemens Ag Schaltungsanordnung zur kompensation von echos in nichtsynchronen traegerfrequenzsystemen
US4268727A (en) * 1979-03-14 1981-05-19 International Telephone And Telegraph Corporation Adaptive digital echo cancellation circuit

Patent Citations (2)

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Publication number Priority date Publication date Assignee Title
US4074086A (en) * 1976-09-07 1978-02-14 Bell Telephone Laboratories, Incorporated Joint adaptive echo canceller and equalizer for two-wire full-duplex data transmission
US4425483A (en) * 1981-10-13 1984-01-10 Northern Telecom Limited Echo cancellation using transversal filters

Non-Patent Citations (2)

* Cited by examiner, † Cited by third party
Title
Mueller, Kurt H., "A New Digital Echo Canceler for Two-Wire Full-Duplex Data Transmission", IEEE Transactions on Communications, vol. COM-24, No. 9, Sep. 1976.
Mueller, Kurt H., A New Digital Echo Canceler for Two Wire Full Duplex Data Transmission , IEEE Transactions on Communications, vol. COM 24, No. 9, Sep. 1976. *

Cited By (8)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US4631475A (en) * 1982-12-21 1986-12-23 Telefonaktiebolaget Lm Ericsson Method of detecting hazardous line voltages over a two-wire line
US4695998A (en) * 1983-07-29 1987-09-22 Siemens Aktiengesellschaft Method and circuit arrangement for the compensation of echo signals
US4707824A (en) * 1983-12-15 1987-11-17 Nec Corporation Method and apparatus for cancelling echo
US4686703A (en) * 1984-03-26 1987-08-11 Cselt-Centro Stude E Laboratori Telecomunicazioni S.P.A. System for bidirectional digital transmission with echo cancellation
US4989221A (en) * 1987-03-30 1991-01-29 Codex Corporation Sample rate converter
US5062102A (en) * 1988-12-01 1991-10-29 Nec Corporation Echo canceller with means for determining filter coefficients from autocorrelation and cross-correlation coefficients
US5852661A (en) * 1995-02-17 1998-12-22 Advanced Micro Devices, Inc. Adaptive echo cancellation used with echo suppression to reduce short and long duration echoes
US6965578B1 (en) * 1998-09-30 2005-11-15 Conexant Systems, Inc. Echo canceling method and apparatus for digital data communication system

Also Published As

Publication number Publication date
MX152441A (es) 1985-07-15
IE53759B1 (en) 1989-02-01
CA1205876A (en) 1986-06-10
MY8600355A (en) 1986-12-31
FI74561B (fi) 1987-10-30
NO832420L (no) 1983-07-01
IT8224025A1 (it) 1984-05-02
DK306483D0 (da) 1983-07-01
AU553372B2 (en) 1986-07-10
ES516965A0 (es) 1984-01-16
JPS58501850A (ja) 1983-10-27
DK306483A (da) 1983-07-01
FI74561C (sv) 1988-02-08
FI832228L (fi) 1983-06-17
SE426764B (sv) 1983-02-07
ES8402483A1 (es) 1984-01-16
EP0092570B1 (en) 1985-08-21
IT1153338B (it) 1987-01-14
CA1191224A (en) 1985-07-30
NO157238C (no) 1988-02-10
EP0092570A1 (en) 1983-11-02
KR840002785A (ko) 1984-07-16
KR860002211B1 (ko) 1986-12-31
WO1983001716A1 (en) 1983-05-11
DE3265650D1 (en) 1985-09-26
NO157238B (no) 1987-11-02
YU244082A (en) 1985-04-30
FI832228A0 (fi) 1983-06-17
IT8224025A0 (it) 1982-11-02
IE822605L (en) 1983-05-02
AU9122282A (en) 1983-05-18
BR8207953A (pt) 1983-10-04

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